How am I able to use multiple operating system images within Docker? - docker

Question

I'm currently using Docker to deploy a development version of a web application. This is the docker-compose.yml file I wrote
version: '3'
services:
nginx:
build:
context: ./docker/nginx
ports:
- "80:80"
volumes:
- .:/var/www/html/acme.com
- ./docker/nginx/acme.com.conf:/etc/nginx/conf.d/acme.com.conf
networks:
- my_network
php:
build:
context: ./docker/php
ports:
- "9000:9000"
volumes:
- .:/var/www/html/acme.com
networks:
- my_network
database:
build:
context: ./docker/database
volumes:
- ./docker/database/acme.sql:/docker-entrypoint-initdb.d/acme.sql
- ./docker/database/remote_access.sql:/docker-entrypoint-initdb.d/remote_access.sql
- ./docker/database/custom.cnf:/etc/mysql/conf.d/custom.cnf
ports:
- "3306:3306"
environment:
MYSQL_DATABASE: ${db_database}
MYSQL_ROOT_PASSWORD: ${db_password}
networks:
- my_network
networks:
my_network:
driver: bridge
Looking more closely at the different context files for nginx and mysql are as follows:
./docker/nginx/Dockerfile
FROM nginx:alpine
./docker/database/Dockerfile
FROM mariadb:latest
So it is evident that nginx image uses the alpine base image.
But what image is mariadb using? I went through the dockerhub website and followed the link to https://github.com/MariaDB/mariadb-docker/blob/db55d2702dfc0102364a29ab00334b6a02085ef9/10.7/Dockerfile
In this file, there is a reference to
FROM ubuntu:focal
Does this mean that my docker container is using the alpine linux base image as well as the ubuntu image? How does it work if I have multiple linux distributions in my container?
What should I do to fix this?
Should I rather install mariadb using a FROM command into alpine linux and build my own docker image?

Answer 1

Imagine your docker compose as a server farm. Each service (nginx, mariadb, ..) would be a physical server running an OS and its software. They are all connected via LAN within the same subnet. Each machine has its own IP address and there is a DNS and DHCP service running for giving the IPs and resolving names (service name = DNS-Alias). And there is a router blocking all connections from other subnets (= your host). Exceptions are configured by port mapping (=forwarding) ports: - 8000:8000.
So you can mix servers with all different OS variants of one type. This is due to the fact that docker is not a real virtual machine but more a VM light using the host OS resources to run the containers. So you can mix all kind of Linux distributions OR Windows versions. Every container uses the OS suiting it goals the best, e.g. Alpine for very small footprint and Ubuntu for much more comfort.